1. Field of the Invention
The present invention relates to a fuel spraying method in a liquid fuel combustion burner used as a heat source of a boiler, a heating furnace and the like, and a liquid fuel combustion burner. More particularly, the present invention relates to a fuel spraying method in a liquid fuel combustion burner having a structure in which a liquid fuel is sprayed together with a fluid flow of air, steam or misty water droplets (hereinafter referred to as "atomization-promoting fluid" or "division promoting fluid") which is mixed in with the liquid fuel, and a liquid fuel combustion burner.
2. Description of the Related Art
A liquid fuel combustion burner having a structure in which a liquid fuel is mixed with an atomization-promoting fluid such as steam or air and this mixed fluid is sprayed from a plurality of injection holes is known.
According to the fuel spraying method adopted for this liquid fuel combustion burner, the liquid fuel to be mixed with the spraying medium is atomized and diffused by the expansion energy generated when an atomization-promoting fluid such as steam or air is injected to a low-pressure side from a high-pressure side.
In regard to the above-mentioned spraying method, two mixing methods are known, an internal mixing method in which the injection quantity is controlled while maintaining a certain difference between the pressure of the atomization-promoting fluid and the pressure of the liquid fuel, and an intermediate mixing method in which the pressure of the liquid fuel is changed while maintaining the pressure of the atomization-promoting fluid at a certain level, whereby the injection quantity is controlled.
The intermediate mixing method is advantageous over the internal mixing method in that the consumption of the atomization-promoting fluid is small and a good atomizing effect is attained.
However, this spraying method is defective in that since the liquid fuel, which is an incompressible fluid, has no substantial dispersing force, the atomization-promoting fluid should be maintained at a high temperature and a high pressure.
As the means for solving this problem, there has been proposed a technique of giving a turning or swirling movement to the atomization-promoting fluid and liquid fuel. The centrifugal force generated by this turning motion promotes atomization and diffusion of the liquid fuel and improves the combustion state (see Japanese Unexamined Utility Model Publication No. 57-145116).
According to this conventional technique, the mixing of the liquid fuel with air and the atomization of the fluid are promoted, not only by the expansion energy generated when steam is injected to a low-pressure side from a high-pressure side, but also by the centrifugal force generated by the turning movement. The liquid fuel also becomes uniformly diffused over a broad range.
Recently, exhaust gas regulations for combustion apparatuses became severe, and reduction of a level of nitrogen oxides (hereinafter referred to as "NO.sub.x ") produced by combustion is therefore an important problem.
Various experiments were done with the above-mentioned conventional liquid fuel combustion burner, and it was found that since the flame layer becomes thick and large and the heat dissipation is degraded, the flame temperature rises, the residence time in a high-temperature zone becomes long and it is difficult to reduce the level of NO.sub.x.
The reason it is difficult to reduce the NO.sub.x level produced by the conventional burner is that in the conventional liquid fuel combustion burner a plurality of injection holes are arranged equidistantly or substantially equidistantly and, thus, the flame layer becomes thick and large and the heat dissipation is degraded.
Moreover, since both of the liquid fuel and the atomization-promoting fluid or division promoting fluid, such as steam are simultaneously turned, the frictional energy between the steam and the liquid fuel is increased.
Accordingly, the consumption of the atomization-promoting fluid, such as steam, increases and, thus, it becomes necessary to elevate the heating temperature of the liquid fuel, with the resultant increase of NO.sub.x in the exhaust gas.